U.S. patent number 9,732,194 [Application Number 14/364,015] was granted by the patent office on 2017-08-15 for graft copolymers of a poly(vinylidene fluoride)-based polymer and at least one type of electrically conductive polymer, and methods for forming the graft copolymers.
This patent grant is currently assigned to Nanyang Technological University. The grantee listed for this patent is Nanyang Technological University. Invention is credited to Vijay Kumar, Pooi See Lee, Meng-Fang Lin.
United States Patent |
9,732,194 |
Lee , et al. |
August 15, 2017 |
Graft copolymers of a poly(vinylidene fluoride)-based polymer and
at least one type of electrically conductive polymer, and methods
for forming the graft copolymers
Abstract
Methods for forming a graft copolymer of a poly(vinylidene
fluoride)-based polymer and at least one type of electrically
conductive polymer, wherein the electrically conductive polymer is
grafted on the poly(vinylidene fluoride)-based polymer are
provided. The methods comprise a) irradiating a poly(vinylidene
fluoride)-based polymer with a stream of electrically charged
particles; b) forming a solution comprising the irradiated
poly(vinylidene fluoride)-based polymer, an electrically conductive
monomer and an acid in a suitable solvent; and c) adding an oxidant
to the solution to form the graft copolymer. Graft copolymers of a
poly(vinylidene fluoride)-based polymer and at least one type of
electrically conductive polymer, wherein the electrically
conductive polymer is grafted on the poly(vinylidene
fluoride)-based polymer, nanocomposite materials comprising the
graft copolymer, and multilayer capacitors comprising the
nanocomposite material are also provided.
Inventors: |
Lee; Pooi See (Singapore,
SG), Kumar; Vijay (Singapore, SG), Lin;
Meng-Fang (Singapore, SG) |
Applicant: |
Name |
City |
State |
Country |
Type |
Nanyang Technological University |
Singapore |
N/A |
SG |
|
|
Assignee: |
Nanyang Technological
University (Singapore, SG)
|
Family
ID: |
48574696 |
Appl.
No.: |
14/364,015 |
Filed: |
December 7, 2012 |
PCT
Filed: |
December 07, 2012 |
PCT No.: |
PCT/SG2012/000461 |
371(c)(1),(2),(4) Date: |
June 09, 2014 |
PCT
Pub. No.: |
WO2013/085467 |
PCT
Pub. Date: |
June 13, 2013 |
Prior Publication Data
|
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|
|
Document
Identifier |
Publication Date |
|
US 20140367036 A1 |
Dec 18, 2014 |
|
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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61568977 |
Dec 9, 2011 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C08G
83/004 (20130101); H01B 3/303 (20130101); C08G
81/024 (20130101); C08F 8/30 (20130101); C08L
101/005 (20130101); C09D 187/005 (20130101); C08J
3/28 (20130101); H01B 3/445 (20130101); H01G
4/30 (20130101); C08F 214/245 (20130101); H01G
4/14 (20130101); H01G 4/306 (20130101); C08F
214/22 (20130101); C08G 73/0266 (20130101); C08F
8/30 (20130101); C08F 214/22 (20130101); C08J
2327/16 (20130101) |
Current International
Class: |
C08F
2/46 (20060101); H01G 4/30 (20060101); H01G
4/14 (20060101); C08F 214/22 (20060101); C08F
8/30 (20060101); C08G 73/02 (20060101); C08F
214/24 (20060101); C08G 81/02 (20060101); C08J
3/28 (20060101); C08G 61/04 (20060101); C09D
187/00 (20060101); C08L 101/00 (20060101); C08G
83/00 (20060101); H01B 3/44 (20060101); H01B
3/30 (20060101) |
Field of
Search: |
;522/126,113,1
;520/1 |
References Cited
[Referenced By]
U.S. Patent Documents
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|
Primary Examiner: Whiteley; Jessica E
Attorney, Agent or Firm: Seed IP Law Group LLP
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of priority of US provisional
application No. 61/568,977 filed on 9 Dec. 2011, the content of
which is incorporated herein by reference in its entirety for all
purposes.
Claims
The invention claimed is:
1. A method for forming a graft copolymer of a poly(vinylidene
fluoride)-based polymer and at least one type of electrically
conductive polymer, wherein the electrically conductive polymer is
grafted on the poly(vinylidene fluoride)-based polymer, the method
comprising the following steps a) through c) in sequential order:
a) irradiating a poly(vinylidene fluoride)-based polymer with a
stream of electrically charged particles; b) forming a solution
comprising the irradiated poly(vinylidene fluoride)-based polymer,
an electrically conductive monomer and an acid in a suitable
solvent; and c) adding an oxidant to the solution to form the graft
copolymer.
2. The method according to claim 1, wherein the electrically
charged particles are electrons.
3. The method according to claim 1, further comprising exposing the
irradiated poly(vinylidene fluoride)-based polymer to oxygen prior
to step (b) to allow formation of peroxides and/or hydroperoxides
on a surface of the polymer.
4. The method according to claim 3, wherein exposing the irradiated
poly(vinylidene fluoride)-based polymer to oxygen comprises placing
the irradiated poly(vinylidene fluoride)-based polymer under
atmospheric conditions for a time period of at least 30
minutes.
5. The method according to claim 1, wherein the poly(vinylidene
fluoride)-based polymer is selected from the group consisting of
poly(vinylidene fluoride) (PVDF),
poly(vinylidenefluoride-co-trifluoroethylene) (PVDF-TrFE),
poly(vinylidene) flouride-hexafluoropropylene (PVDF-HEP),
poly(vinylidene fluoride-chlorotrifluoroethylene) (PVDF-CTFE),
poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene)
(PVDF-TrFE-CFE), derivatives thereof, and mixtures thereof.
6. The method according to claim 5, wherein the poly(vinylidene
fluoride)-based polymer comprises poly(vinylidene fluoride).
7. The method according to claim 1, wherein the electrically
conductive monomer is selected from the group consisting of
aniline, pyrrole, thiophene, bisthiophene, furan, para-phenylene,
phenylene vinylene, para-phenylene sulfide, thienylene-vinylene,
acetylene, indole, carbazole, imidazole, pyridine, pyrene, azulene,
naphthalene, derivatives thereof, and mixtures thereof.
8. The method according to claim 7, wherein the electrically
conductive monomer comprises aniline.
9. The method according to claim 1, wherein the acid is selected
from the group consisting of hydrochloric acid, sulfuric acid,
dodecylbenzenesulfonic acid, naphthalene-2-sulfonic acid,
poly(4-syyrenesufonic acid), and mixtures thereof.
10. The method according to claim 1, wherein the oxidant is
selected from the group consisting of ammonium peroxydisulfate
(APS), potassium biiodate (KH(IO.sub.3).sub.2), iron (III)
chloride, and mixtures thereof.
11. The method according to claim 1, wherein adding the oxidant to
the solution comprises dripping the oxidant in a drop wise fashion
into the solution.
12. The method according to claim 1, further comprising blowing air
into the solution after step c) to quench the polymerization
reaction.
13. A method of forming a multilayer capacitor, the method
comprising a) coating a layer of a first metal on at least a
portion of one surface of a nanocomposite material comprising a
graft copolymer formed by the method of claim 1, the graft
copolymer being of a poly(vinylidene fluoride)-based polymer and at
least one type of electrically conductive polymer, wherein the
electrically conductive polymer is grafted on the poly(vinylidene
fluoride)-based polymer; (b) arranging a plurality of the
metal-coated nanocomposite material formed in (a) in a stack such
that the metal-coated surfaces do not contact each other but face
the same direction; and (c) coating a layer of a second metal on at
least a portion of each of two external surfaces of the stack
opposing each other and lateral to the external surface of the
stack with the layers of first metal coated thereon to form the
multilayer capacitor.
14. The method according to claim 13, wherein the plurality of
metal-coated nanocomposite materials comprises three or more
metal-coated nanocomposite materials.
15. The method according to claim 13, wherein the first metal is
selected from the group consisting of platinum, silver, gold,
aluminium, nickel, copper, and alloys thereof.
Description
TECHNICAL FIELD
The invention relates to graft copolymers of a poly(vinylidene
fluoride)-based polymer and at least one type of electrically
conductive polymer, as well as methods to form the graft
copolymer.
BACKGROUND
Capacitors are widely used in application areas, such as industrial
appliances, medicine, automobiles, aircraft, space, power supply
circuits, computers, power electronics, and energy storage. Of the
myriad of applications, electronic components and energy storage
devices, in particular, require the use of materials with high
performance dielectric properties.
Electroactive polymers have shown promise for energy storage due to
their excellent properties. Using poly(vinylidene fluoride) (PVDF)
as an example, it is an electroactive thermoplastic polymer having
excellent chemical resistance, good stability, high volume
resistivity, low water absorption rate with potential pyroelectric,
piezoelectric, and ferroelectric properties. Specifically, PVDF
provides the best polymer ferroelectric property and the highest
dielectric constant among all polymers. Notwithstanding the above,
its dielectric constant is far lower than that of non-polymer
counterpart such as dielectric ceramics. To qualify PVDF for use in
high charge-storage capacitors or electrostriction system for
artificial muscles, for example, there is a need to improve the
dielectric constant of PVDF.
A number of methods have been used to improve the dielectric
properties of PVDF. For example, the dielectric properties of PVDF
may be improved by incorporating perovskite ceramic particles of
high dielectric constants into the polymer. Although the dielectric
constant of such polymer-ceramic composites has been found to be
higher than that of pristine PVDF, polymers, unsatisfactory
dielectric loss and low breakdown field strength of the composites
have limited their application. Furthermore, high volume content of
the ceramic fillers in the polymer contributes to a loss in
flexibility, and lowers quality of the composites.
Apart from the use of perovskite ceramic particles, other additives
such as metal particles and carbon nanotubes have been introduced
into the polymers by physical blending. However, this approach
suffers from drawbacks such as adverse excessive agglomeration of
fillers due to the incompatibility of these fillers with the
polymer matrix, which in turn leads to dielectric loss and
reduction of breakdown field.
In view of the above, there is a need for a material with improved
dielectric properties which addresses at least one of the
above-mentioned problems.
SUMMARY OF THE INVENTION
In a first aspect, the invention refers to a method for forming a
graft copolymer of a poly(vinylidene fluoride)-based polymer and at
least one type of electrically conductive polymer, wherein the
electrically conductive polymer is grafted on the poly(vinylidene
fluoride)-based polymer. The method comprises a) irradiating a
poly(vinylidene fluoride)-based polymer with a stream of
electrically charged particles; b) forming a solution comprising
the irradiated poly(vinylidene fluoride)-based polymer, an
electrically conductive monomer and an acid in a suitable solvent;
and c) adding an oxidant to the solution to form the graft
copolymer.
In a second aspect, the invention refers to a graft copolymer of a
poly(vinylidene fluoride)-based polymer and at least one type of
electrically conductive polymer, wherein the electrically
conductive polymer is grafted on the poly(vinylidene
fluoride)-based polymer, formed by a method according to the first
aspect.
In a third aspect, the invention refers to a graft copolymer of a
poly(vinylidene fluoride)-based polymer and at least one type of
electrically conductive polymer, wherein the electrically
conductive polymer is grafted on the poly(vinylidene
fluoride)-based polymer.
In a fourth aspect, the invention refers to a method for forming a
nanocomposite material. The method comprises (a) dissolving the
graft copolymer according to the second aspect or the third aspect
in an organic solvent to form a solution of the graft copolymer;
(b) coating the solution on a substrate; and (c) drying the
solution-coated substrate to form a nanocomposite material on the
substrate.
In a fifth aspect, the invention refers to a nanocomposite
comprising the graft copolymer according to the second aspect or
the third aspect, or formed by a method according to the fourth
aspect.
In a sixth aspect, the invention refers to a method of forming a
multilayer capacitor. The method comprising (a) coating a layer of
a first metal on at least a portion of one surface of the
nanocomposite material according to the fifth aspect; (b) arranging
a plurality of the metal-coated nanocomposite material formed in
(a) in a stack such that the metal-coated surfaces do not contact
each other but face the same direction; and (c) coating a layer of
a second metal on at least a portion of each of two external
surfaces of the stack opposing each other and lateral to the
external surface of the stack with the layers of first metal coated
thereon to form the multilayer capacitor.
In a seventh aspect, the invention refers to a multilayer capacitor
comprising a nanocomposite according to the fifth aspect, or formed
by a method according to the sixth aspect.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be better understood with reference to the
detailed description when considered in conjunction with the
non-limiting examples and the accompanying drawings, in which:
FIG. 1 is a schematic diagram depicting synthesis of
poly(vinylidene fluoride) (PVDF)-graft-polyaniline (PANI) via
oxidative radical polymerization according to various embodiments.
In the embodiment shown in (A), PVDF is contacted with aniline in
the presence of an oxidant such as ammonium persulfate (APS) and an
acid such as hydrochloric acid (HCl) to form a graft copolymer of
PVDF and polyaniline, as shown in (B).
FIG. 2 is a schematic diagram depicting surface modification of
PVDF polymer powder according to various embodiments. In the
embodiment shown in (A), PVDF is irradiated with a stream of
electrically charged particles such as electrons. As shown in (B),
the irradiated PVDF polymer is exposed to oxygen to allow formation
of peroxides on a surface of the polymer. In various embodiments,
presence of moisture on the surface of the polymer and or in the
atmosphere results in formation of hydroperoxides on the polymer
surface, such as that shown in (C).
FIG. 3 is a schematic diagram depicting primary sulfate ion radical
generation according to various embodiments. In (A), oxidizing
action of APS is shown. In (B), APS acts as an initiator and
primary sulfate ion radicals are generated.
FIG. 4 is a schematic diagram depicting formation of secondary
polyaniline ion radical (PANI*) according to various embodiments,
where (A) initiation; (B) propagation; and (C) termination are
shown.
FIG. 5 is a schematic diagram depicting graft copolymerization of
polyaniline (PANI) onto PVDF according to various embodiments. In
(A), the irradiated PVDF generates radicals which react with
secondary polyaniline ion radical (PANI*) as shown in (B). In (C),
the polyaniline ion radical may react with another polyaniline ion
radical to form a homopolymer as a side reaction. In (D),
PVDF-g-PANI graft copolymer is formed.
FIGS. 6 (A) and (B) are photographs of (i) pristine PVDF (White)
and (ii) PVDF-g-PANI copolymer film.
FIG. 7 shows (A) Fourier Transform Infra-red (FTIR) spectra of (i)
pristine PVDF, (ii) polyaniline, and (iii) PVDF-g-PANI polymer
powder; (B) TGA thermograms of (i) pristine PVDF, (ii) polyaniline,
and (iii) PVDF-g-PANI polymer powder; and (C) DSC analysis of (i)
pristine PVDF, (ii) polyaniline, and (iii) PVDF-g-PANI polymer
powder.
FIG. 8 is a graph showing electrical conductivity of pristine PVDF
and PVDF-g-PANI films.
FIG. 9 are graphs showing (A) dependence of the dielectric constant
of pristine PVDF and PVDF-g-PANI (with optimum grafting) on the
frequency at room temperature; and (B) effect of the percentage of
grafting of polyaniline onto the dielectric constant of PVDF at
room temperature measured at 1 KHz.
FIG. 10 are graphs showing (A) dependence of dielectric losses on
the frequency at room temperature; (B) effect of the percentage of
grafting of polyaniline onto the dielectric losses of PVDF at room
temperature measured at 1 KHz.
FIG. 11 are graphs showing (A) dependence of the dielectric
constant of pristine PVDF and PVDF-g-PANI on temperature at the
frequency 10 KHz; and (B) dependence of the dielectric losses of
pristine PVDF and PVDF-g-PANI on temperature at the frequency 10
KHz.
FIG. 12 is a schematic diagram of a 3-layered PVDF-g-PANI
multilayer polymer (MLP) capacitor according to various
embodiments.
FIG. 13 is a graph showing dependences of capacitance of
PANI-g-PVDF Multilayer Polymer (MLP) Capacitor on frequency
measured at room temperature from 10.sup.2 to 10.sup.6 Hz.
FIG. 14 is a graph showing discharge time comparison for various
polymer samples measured at a circuit resistance of 2.7 KO, a)
PVDF, b) PANI-g-PVDF 3.6%, c) PANI-g-PVDF 5.2%, d) PANI-g-PVDF
7.72% e) PANI-g-PVDF 9.7%.
FIG. 15(A) a cross-sectional view of a multilayer capacitor 100
according to an embodiment of the sixth aspect. A layer of a first
metal 101 is coated on at least a portion of a surface of the
nanocomposite material 103. A plurality of the metal-coated
nanocomposite material formed is arranged in a stack such that the
metal-coated surfaces do not contact each other but face the same
direction. Layers of a second metal 105 is coated on two external
surfaces of the stack opposing each other and lateral to the
external surface of the stack with the layers of first metal coated
thereon to form the multilayer capacitor. FIG. 15(B) is a schematic
diagram depicting effective resistance R of the multilayer
capacitor, which may be calculated using resistance of individual
layers R.sub.1 and R.sub.2, and the equation
##EQU00001## This results in a low value of R as compared to
R.sub.1 and R.sub.2.
DETAILED DESCRIPTION OF THE INVENTION
In a first aspect, the present invention refers to a method for
forming a graft copolymer of a poly(vinylidene fluoride)-based
polymer and at least one type of electrically conductive polymer,
wherein the electrically conductive polymer is grafted on the
poly(vinylidene fluoride)-based polymer.
The method of the first aspect offers a number of advantages such
as improved compatibility of the electrically conductive polymers
so formed with the poly(vinylidene fluoride)-based polymer matrix
due to covalent bond formation between the electrically conductive
polymers and the poly(vinylidene fluoride)-based polymer.
Furthermore, a greater amount of electrically conductive
monomerspolymers may be incorporated at different reactive sites on
the poly(vinylidene fluoride)-based polymer. In so doing, a large
number of nanocapacitors, in the form of arrays of rationally
positioned nanocapacitors having improved dielectric properties,
may be obtained. This is particularly advantageous since such
arrays of rationally positioned nanocapacitors cannot be obtained
using traditional blending methods. One reason for this is
aggregation of electrically conductive polymers in nanocomposites
that are prepared using traditional blending methods. The
aggregated polymers form conductive paths which results in high
leakage current in devices formed using the nanocomposites. By
grafting the electrically conductive polymer on poly(vinylidene
fluoride)-based polymer according to various embodiments of the
invention, aggregation of the electrically conductive polymer in
the nanocomposites is prevented. This translates into improved
performance in the devices formed using the nanocomposites.
Furthermore, early confrontation with percolation threshold may be
avoided.
As used herein, the term "graft copolymer" refers to a copolymer
having a backbone or main chain to which side chains of a different
chemical composition are attached at various positions along the
backbone. For example, the backbone may be formed of a first
polymer and the side chains of a second polymer, wherein the first
polymer and the second polymer have different chemical
compositions. The side chains may be attached at various positions
along the backbone by covalent bonding to form the graft
copolymer.
In various embodiments, the backbone or the main chain of the graft
copolymer is formed of a poly(vinylidene fluoride)-based polymer.
Examples of poly(vinylidene fluoride)-based polymer include, but
are not limited to, poly(vinylidene fluoride) (PVDF),
poly(vinylidenefluoride-co-trifluoroethylene) (PVDF-TrFE),
poly(vinylidene) flouride-hex afluoropropylene (PVDF-HEP),
poly(vinylidene fluoride-chlorotrifluoroethylene) (PVDF-CTFE),
poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene)
(PVDF-TrFE-CFE), derivatives thereof and mixtures thereof. In
various embodiments, the poly(vinylidene fluoride)-based polymer
comprises or consists essentially of poly(vinylidene fluoride).
The side chains are formed from at least one type of electrically
conductive polymer. The term "electrically conductive polymer"
refers to a polymer or an oligomer which is inherently or
intrinsically capable of electrical conductivity. Besides
electrically conductive polymers, the term "electrically conductive
polymer" refers also to semi-conductive polymers. Examples of
electrically conductive polymers include, but are not limited to,
polyaniline, polypyrrole, polythiopene, polyacetylene, polypyrene,
polyphenylene, and polyacene. Two or more electrically conductive
polymers may be used to form the side chains of the graft
copolymer. In various embodiments, the electrically conductive
polymer comprises or consists essentially of polyaniline. In one
embodiment, the electrically conductive polymer consists of
polyaniline. The use of polyaniline is particularly advantageous,
as it is chemically stable at room temperature conditions, and may
be synthesized easily using chemical or electrochemical
methods.
The electrically conductive polymer may be derived from an
electrically conductive monomer by polymerization of the monomers,
for example. In various embodiments, the electrically conductive
polymer is derived from electrically conductive monomers selected
from the group consisting of aniline, pyrrole, thiophene,
bisthiophene, furan, para-phenylene, phenylene vinylene,
para-phenylene sulfide, thienylene-vinylene, acetylene, indole,
carbazole, imidazole, pyridine, pyrene, azulene, naphthalene,
derivatives thereof, and mixtures thereof. In various embodiments,
the electrically conductive polymer is derived from monomers
comprising or consisting essentially of aniline.
The method of the first aspect includes irradiating a
poly(vinylidene fluoride)-based polymer with a stream of
electrically charged particles. As used herein, the term
"electrically charged particles" refers to particles, molecules,
ions or sub-atomic particles that carry an electric charge.
Examples of electrically charged particles include, but are not
limited to, positively and negatively charged particles, positively
and negatively charged ions, electrons, and protons. In various
embodiments, the electrically charged particles are electrons.
Accordingly, the stream of electrically charged particles may be in
the form of a high energy ionizing radiation, such as electron beam
irradiation.
The stream of electrically charged particles should have sufficient
energy to penetrate the mass of poly(vinylidene fluoride)-based
polymer being irradiated to the extent specified. For example, the
stream of electrically charged particles may be in the form of
electrons beamed from an electron generator having an accelerating
potential of 500 to 4,000 kilovolts. In various embodiments,
ionizing radiation doses of about 0.1 to about 2 megarads ("Mrad"),
such as about 1 to 2 Mrad, or about 1.6 Mrad are used. In various
embodiments, irradiating the poly(vinylidene fluoride)-based
polymer with a stream of electrically charged particles is carried
out under vacuum.
The irradiation may be carried out to create active sites on the
polymer backbone, which are then used to initiate graft
polymerization of the electrically conductive monomer to produce
graft copolymers. An exemplary embodiment is shown in FIG. 2(A),
whereby a radical is formed in the molecule of PVDF by irradiating
PVDF with a stream of electrically charged particles.
Besides using irradiation, chemical methods in which the polymer
backbone is treated with organic chemical compounds capable of
generating free radicals may also be used to generate active sites
on the polymer backbone. For example, in the chemical method, an
organic chemical compound capable of generating free radicals, such
as a peroxide or azo compound, may be decomposed in the presence of
the backbone polymer with formation of free radicals. The free
radicals form the active grafting sites on the polymer, and
initiate polymerization of the monomers at these sites. Even though
chemical methods may also be used to generate the active sites on
the poly(vinylidene fluoride)-based backbone, the use of
irradiation to create the active sites is advantageous in that the
graft copolymer has a higher grafting efficiency as compared with
that prepared by chemical methods.
The irradiated poly(vinylidene fluoride)-based polymer may be
exposed to oxygen to allow formation of peroxides and/or
hydroperoxides on a surface of the polymer by reaction of the free
radicals in the polymer with oxygen. Formation of the peroxides
and/or hydroperoxides allows chemical-induced graft
copolymerization of the electrically conductive monomers on the
poly(vinylidene fluoride)-based polymer, of which an exemplary
embodiment is shown in FIG. 2(B). Concentration of peroxide groups
and/or hydroperoxides groups formed on the polymer may be
controlled for example, by varying the irradiation dose, and/or the
amount of oxygen to which the polymer is exposed after irradiation.
On a similar note, the amount of time required for forming the
peroxides and/or hydroperoxides may be determined by a person
skilled in the art, and may depend, for example, on the amount of
oxygen present, the irradiation dose, and the temperature.
In various embodiments, the irradiated poly(vinylidene
fluoride)-based polymer may be exposed to oxygen by placing the
polymer under atmospheric conditions for a time period of at least
30 minutes, such as 40 minutes, 50 minutes, or an hour. As used
herein, the term "atmospheric conditions" means approximately
25.degree. C. in atmospheric air containing approximately 21%
oxygen by volume, with trace amounts of water vapor.
The method of the first aspect includes forming a solution
comprising the irradiated poly(vinylidene fluoride)-based polymer,
an electrically conductive monomer and an acid in a suitable
solvent.
Examples of poly(vinylidene fluoride)-based polymer and
electrically conductive monomers that may be used have already been
described above. An acid may be added to adjust the pH of the
solution. In various embodiments, the acid may be hydrochloric
acid, sulfuric acid, dodecylbenzenesulfonic acid,
naphthalene-2-sulfonic acid, poly(4-syyrenesufonic acid), or
mixtures thereof. In some embodiments, the acid comprises or
consists essentially of hydrochloric acid.
A solvent may be used in order to decrease the viscosity during
processing and/or to facilitate polymerization by allowing stirring
of the solution. Generally, the solvent may be any substance which
is liquid, able to dissolve the irradiated poly(vinylidene
fluoride)-based polymer and the electrically conductive monomer,
and inert in that it does not react with the reactants and does not
adversely affect the reaction. For example, the solvent may be an
aqueous solution such as water, or an aqueous buffer solution such
as saline. In various embodiments, the solvent comprises or
consists essentially of distilled water.
To allow better mixing of the irradiated poly(vinylidene
fluoride)-based polymer, electrically conductive monomer and acid
in the solvent, the solution may be agitated, for example, by
stirring or sonicating, and at a temperature that is suitable for
increasing the rate at which the reactants dissolve in the solvent.
In various embodiments, an inert gas such as nitrogen is purged
into the solution to prevent oxidation of the reactants during
grafting reaction.
The method of the first aspect further includes adding an oxidant
to the solution to form the graft copolymer. To form the graft
copolymer, the graft polymerization reaction may be a free radical
polymerization reaction initiated by exposing the PVDF polymer
backbone to an oxidant. Examples of oxidant include, but are not
limited to, oxygen, air, H.sub.2O.sub.2 or metal oxides such as
manganese oxide and vanadium oxide. In various embodiments, the
oxidant is selected from the group consisting of ammonium
peroxydisulfate (APS), potassium biiodate (KH(IO.sub.3).sub.2),
iron (III) chloride, and mixtures thereof. In various embodiments,
the oxidant comprises or consists essentially of ammonium
peroxydisulfate (APS). The oxidant may be added to the solution by
dripping the oxidant in a drop wise fashion into the solution. In
so doing, the rate of polymerization for forming the graft
copolymer may be carried out in a controlled manner, and may
translate into improvements in uniformity of the grafting reaction.
The rate of polymerization for forming the graft copolymer may
alternatively be controlled, for example, by using differing
extents of agitation, and/or by controlling the temperature of the
solution.
The method may further comprise blowing air into the solution to
quench the polymerization reaction. By blowing air into the
solution, the radicals of the polymers recombine with oxygen in the
air to lose their functionality as initiation points of
polymerization.
In various embodiments, the method may further include
precipitating the graft copolymer formed with a reagent. For
example, ethanol may be used to precipitate the graft copolymer.
The precipitated polymer may be subjected to further purification
steps to remove unwanted side products. For example, m-cresol,
which is a good solvent for homopolymer of polyaniline, may be used
to wash the graft copolymer so as to remove the polyaniline.
In further aspects, the invention refers to a graft copolymer of a
poly(vinylidene fluoride)-based polymer and at least one type of
electrically conductive polymer, wherein the electrically
conductive polymer is grafted on the poly(vinylidene
fluoride)-based polymer, which may be formed by the method
according to the first aspect. Examples of an electrically
conductive polymer that may be used have already been described
above.
By grafting the electrically conductive polymer as side chains to
the poly(vinylidene fluoride)-based polymer backbone, there is
improved compatibility of the electrically conductive polymers with
the poly(vinylidene fluoride)-based polymer matrix due to covalent
bond formation.
In a fourth aspect, the invention refers to a method for forming a
nanocomposite material. The method includes dissolving the graft
copolymer according to the second aspect or the third aspect in an
organic solvent to form a solution of the graft copolymer. Suitable
organic solvents that may be used include N,N-dimethylformamide
(DMF), N,N-dimethylacetamide (DMA), butanone, acetone,
N-Methyl-2-pyrrolidone (NMP), tetrahydrofuran (THF), dimethyl
sulfoxide (DMSO), and mixtures thereof.
The method includes coating the solution on a substrate. Generally,
any thin film coating method may be used. Examples of thin film
coating methods include, but are not limited to, chemical vapour
deposition, sol gel deposition, spin coating, screen printing, tip
coating, atomic layer deposition, roller coating, layer by layer
coating, and pulsed laser deposition. In various embodiments, the
solution is coated on the substrate by drop casting, spin coating,
roller coating, layer by layer coating, or tape casting.
After coating the solution on the substrate, the solution-coated
substrate is dried to form a nanocomposite material on the
substrate. In drying the solution-coated substrate, at least some
of the organic solvent that is used to dissolve the graft copolymer
may be removed, leaving behind the nanocomposite material on the
substrate.
Depending on the type of application for example, an additive may
optionally be dissolved or dispersed in the solution prior to
coating of the solution on the substrate. For example, to enhance
the electrical conductivity of the nanocomposite formed, the
additive may comprise an electrically conductive material, such as
perovskite ceramic nanoparticles, metal nanoparticles and carbon
nanotubes. The additive may also assume other shapes such as
nanoflakes and nanosheets. To allow better mixing or dispersion of
the additive in the solution, the solution may be agitated by
stirring or sonicating, and in some embodiments, heated to increase
the rate at which the additive dissolves in the solvent.
Thickness of the nanocomposite material layer may generally range
from a few atomic layers or few monolayers, or from nanometers to a
few micrometers. Typically, the thickness of the nanocomposite
material layer ranges from about 5 nm to about 100 .mu.m, such as
about 5 nm to about 100 nm, about 100 nm to about 500 nm, about 1
.mu.m to about 5 .mu.m, about 10 .mu.m to about 50 .mu.m, or about
10 .mu.m to about 100 .mu.m.
The invention refers in a fifth aspect to a nanocomposite
comprising the graft copolymer according to the second aspect or
the third aspect, or formed by the method according to the fourth
aspect. The nanocomposite may, for example, be used as a dielectric
material in a capacitor.
Accordingly, in a sixth aspect, the invention refers to a method of
forming a multilayer capacitor. The term "multilayer capacitor" may
also be referred herein as a "multilayer polymer capacitor" (MLP).
The method comprises coating a layer of a first metal on at least a
portion of one surface of the nanocomposite material according to
the fifth aspect; arranging a plurality of the metal-coated
nanocomposite materials in a stack such that the metal-coated
surfaces do not directly contact each other but face the same
direction; and coating a layer of a second metal on at least a
portion of each of two external surfaces of the stack opposing each
other and lateral to the external surface of the stack having the
layers of first metal coated thereon to form the multilayer
capacitor. In other words, the plurality of nanocomposite materials
are placed on top of each other in the same orientation, i.e. the
metal-coated layer faces upward or downward, and at two opposing
lateral or side walls relative to the external surface of the stack
where the metal-coated layer is present, a layer of a second metal
is deposited on at least a portion of the surface of each of the
two opposing lateral walls of the stack.
The term "capacitor" refers generally to an electronic device or
component that stores an electrical charge. Typically, a capacitor
includes two conductive electrodes or plates separated by an
insulator or a dielectric. Each of the two conductive electrodes
stores an electrical charge at the surface of the electrode at the
boundary with the dielectric, whereby the electrical charge at each
electrode is opposite to that stored on the other electrode.
In its simplest form, a capacitor contains two conducting plates
that sandwich an insulating medium. Such a capacitor configuration
may also be referred to as a simple plate capacitor. The
capacitance C of the capacitor may be calculated by the equation
C=KeA/d
wherein K denotes the dielectric constant of the insulating medium;
e denotes the permittivity of free space; A denotes the effective
area common to the two conducting plates; and d denotes the
distance separating the two plates.
In line with the above, to obtain a capacitor with high
capacitance, the dielectric constant of the dielectric material and
the effective area common to the two conducting plates should be
high, while the distance separating the two plates i.e. thickness
of the dielectric material should be small. In application
therefore, when the number of layers of nanocomposite material in
the stack increases, the effective area that is common to the
conducting plates increases, thereby resulting in an increase in
capacitance.
An example of a multilayer capacitor is depicted in FIG. 15, which
is a cross-sectional view of a multilayer capacitor 100 according
to an embodiment. A layer of a first metal 101 is coated on at
least a portion of a surface of the nanocomposite material 103. A
plurality of the metal-coated nanocomposite material formed is
arranged in a stack such that the metal-coated surfaces do not
contact each other but face the same direction. Layers of a second
metal 105 is coated on two external surfaces of the stack, opposing
each other and lateral to the external surface of the stack with
the layers of first metal coated thereon to form the multilayer
capacitor.
The number of layers of metal-coated nanocomposite material to form
the capacitor may depend on the type of application hence the
capacitance level required. In various embodiments, the number of
layers of metal-coated nanocomposite material is at least two, such
as three, four or five layers. In some embodiments, the number of
metal-coated nanocomposite material is three or more. In one
embodiment, the number of metal coated nanocomposite material is
three.
In various embodiments, the nanocomposite material to be stacked
may be prepared using a graft copolymer according to the second or
the third aspect or hybrid layers formed from a mixture of polymer
and nanocomposite, to reduce leakage current. An exemplary
embodiment is shown in FIG. 12, where "A" denotes the graft
copolymer layer and "B" denotes a hybrid layer.
The first metal and the second metal may be the same or different.
In various embodiments, the first metal is selected from the group
consisting of platinum, silver, gold, aluminium, nickel, copper,
and alloys thereof. The second metal may be selected from the group
consisting of platinum, silver, gold and alloys thereof.
The second metal may be coated on two external surfaces of the
stack, in which the two external surfaces are opposing each other,
and are lateral to the surfaces of the stack having the layers of
first metal coated thereon. In some embodiments, the two external
surfaces are at least substantially perpendicular to the surfaces
of the stack having the layers of first metal coated thereon. The
second metal may be formed on the external surfaces of the stack
such that they are in electrical contact with the layers of first
metal to form the multilayer capacitor. Each individual layer of
the stack may be arranged at least substantially parallel to one
another such as that shown in FIG. 15(A).
FIG. 15(B) is a schematic diagram depicting effective resistance R
of the multilayer capacitor, which may be calculated using
resistance of individual layers R.sub.1 and R.sub.2, and the
equation
##EQU00002## This results in a low value of R as compared to
R.sub.1 and R.sub.2.
The second metal may be coated using any suitable thin film coating
method. Examples of thin film coating method have already been
mentioned above.
In a seventh aspect, the invention refers to a multilayer capacitor
comprising the nanocomposite according to the fifth aspect, or
formed by the method according to the sixth aspect.
The multilayer capacitor may include at least two nanocomposite
layers. In various embodiments, the number of nanocomposite layers
is three or more. As mentioned above, the presence of multilayers
of the nanocomposite which function as dielectrics can
substantially increase the dielectric strength of the capacitor,
thereby enhancing the energy storage ability of the capacitor.
The invention illustratively described herein may suitably be
practiced in the absence of any element or elements, limitation or
limitations, not specifically disclosed herein. Thus, for example,
the terms "comprising", "including", "containing", etc. shall be
read expansively and without limitation. Additionally, the terms
and expressions employed herein have been used as terms of
description and not of limitation, and there is no intention in the
use of such terms and expressions of excluding any equivalents of
the features shown and described or portions thereof, but it is
recognized that various modifications are possible within the scope
of the invention claimed. Thus, it should be understood that
although the present invention has been specifically disclosed by
preferred embodiments and optional features, modification and
variation of the inventions embodied therein herein disclosed may
be resorted to by those skilled in the art, and that such
modifications and variations are considered to be within the scope
of this invention.
The invention has been described broadly and generically herein.
Each of the narrower species and subgeneric groupings falling
within the generic disclosure also form part of the invention. This
includes the generic description of the invention with a proviso or
negative limitation removing any subject matter from the genus,
regardless of whether or not the excised material is specifically
recited herein.
Other embodiments are within the following claims and non-limiting
examples. In addition, where features or aspects of the invention
are described in terms of Markush groups, those skilled in the art
will recognize that the invention is also thereby described in
terms of any individual member or subgroup of members of the
Markush group.
EXPERIMENTAL SECTION
Advantages of graft copolymer formed herein include incorporation
of an electrically conductive polymer such as polyaniline into a
poly(vinylidene fluoride)-based polymer. This allows covalent
bonding of polyaniline to PVDF. The graft copolymer may be used to
form multilayer polymer capacitor device for the benefit of high
energy density capacitors. From the results obtained, it can be
seen that there is significant improvement in terms of dielectric
properties for high performance capacitors, which may be attributed
to the following: (1) dipole density, which is characterized by the
total number of dipoles per unit volume, and (2) the ability of the
dipoles to follow the reversal of polarity with applied electrical
field, i.e., the mobility of the dipole, which in turn depends upon
the mobility of the charges on the polymer chains to which the
dipoles are attached. The dielectric constant obtained in the
PVDF-g-PANI copolymer is much higher than that in other polymer
system reported thus far. In addition, discharge time of grafted
polymer is much superior to that of bulk ceramic material.
Example 1: Materials
PVDF (M=420,000) pellets with a diameter of 0.1 mm used in this
work were supplied by Solvay Solexis Inc. Conductive PANI monomer
(aniline) was received from SigmaAldrich. Aniline was distilled
prior to use and ammonium persulphate was used without further
purification. Concentrated hydrochloric acid, m-cresol, acetone,
N-Methylpyrrolidone (NMP) and methanol were supplied by
(SigmaAldrich Chemie GmbH, Germany). Deionized water was used for
all the reactions. All the materials mentioned above were purified
whenever necessary.
Example 2: Preparation of Polymer Films
The PVDF and PVDF-g-PANI powders in the ratio 60 mg ml were
dissolved homogeneously in the solvent of N, N-Dimethylformamide
(DMF). The solution was stirred and heated at 60.degree. C.
simultaneously for twelve hours in order to have uniform mixing to
form the film precursors. Subsequently, the PVDF PVDF-g-PANI films
were tape casted onto glass substrate using the precursors and
dried in air at 60.degree. C. for twelve hours and in vacuum at
50.degree. C. for twelve hours to remove DMF. The film thickness
was measured using a micrometer accurate to 0.001 mm while the
weight of the samples was measured using a laboratory scale
accurate to 0.01 mg.
Example 3: Fabrication of Multilayer Capacitor
For testing the electrical properties, two types of capacitor were
designed. Single layer parallel-plate capacitors were fabricated by
using a sputter coater to deposit 50 nm thick Pt through an array
of 1 mm.sup.2 circular holes to form the top electrode, and on the
other side of PVDF-g-PANI films to form the back electrode. A
multilayer polymer (MLP) capacitor, comprising three layers of Pt
sputter coated PVDF-g-PANI sheets was fabricated as shown in FIG.
12.
Example 4: Instrumentation
The dielectric properties of samples were measured using a Keithley
4200 semiconductor parameter analyzer and a HP4284 LCR meter in the
frequency range of 100 Hz to 1 MHz at different temperatures. A
high-voltage instrument (CS2674A) was used to measure the
electrical breakdown strength. The voltage was applied continuously
on the sample until it broke down.
Various samples of pristine PVDF and PVDF-g-PANI copolymers were
characterized using Fourier transform infrared spectroscopy (FTIR),
Thermogravimetric Analysis (TGA) and Differential Scanning
calorimetry (DSC).
Example 5: Activation of PVDF Polymer Samples
PVDF powder samples after proper drying were placed in plastic bags
and purged with nitrogen for ten minutes to remove oxygen prior to
irradiation. The PVDF samples sealed in plastic bags were
irradiated in vacuum using the Energy Sciences Inc. (ESI) Electron
Beam Accelerator at room temperature (25.degree. C.). The radiation
dose of 1.6 Mrad was used at fixed speed of 18 MPM in order to have
maximum exposure of radiation to the polymer. After irradiation,
the irradiated samples were exposed to the atmosphere at room
temperature (25.degree. C.) for at least 30 minutes to facilitate
the formation of surface peroxides and hydro peroxides for the
subsequent chemical-induced graft copolymerization experiments.
Example 6: Chemical Synthesis of PVDF-Graft-Polyaniline Copolymers
(PVDF-g-PANI)
FIG. 1 is a schematic diagram depicting synthesis of
poly(vinylidene fluoride) (PVDF)-graft-polyaniline (PANI) via
oxidative radical polymerization according to various
embodiments.
Chemical induced graft copolymerization was carried out in a 500 mL
tri-necked flask equipped with thermometer, condenser and magnetic
stirrer with heating mantle. In a typical reaction, 0.5 g
pre-irradiated PVDF (electron beam irradiation conditions, viz. the
radiation dose, conveyer speed, and maximum accelerating voltage
were fixed at 1.6 Mrad, 18 MPM, 175 kV respectively), aniline (0.1
mol/L) and hydrochloric acid (0.9 mol/L) were added in 50 mL of
distilled water, and nitrogen was purged into the solution for
thirty minutes. The mixture was thermo-stated at temperature of
55.+-.5.degree. C. under nitrogen (N.sub.2) atmosphere. After
thirty minutes, definite amount of 0.08 mol/L APS dissolved in 10
mL distilled water was added drop wise via a syringe through a
rubber septum and this was taken as zero time.
The grafting reaction was carried out at known fixed temperature
with magnetic stirring. After a further 5 hours of continuous
stirring, the mixture became a green solution. At the end of
reaction time, the reaction was arrested by blowing air into the
reaction flask to freeze further reactions. The reaction mixture
was then precipitated with absolute ethanol. The graft copolymers
obtained were collected by filtration and thoroughly washed with
distilled water and ethyl ether, followed by acetone for complete
separation of ungrafted PANI from the grafted material.
In order to make sure that all the polyaniline homopolymer is
removed; the grafted copolymer was washed several times with
m-cresol (good solvent for homopolymer of polyaniline) followed by
Soxhlet extraction in the m-cresol for 72 hours. Finally, the
product was washed with 1 M HCl solution and dried at 40.degree. C.
for 24 hours under vacuum. The percentage grafting was calculated
from increase in the weight of original polymer using the method
reported earlier.
Example 7: Mechanism for Graft Copolymerization Synthesis
Grafting of aniline onto PVDF polymer is suggested to follow the
mechanism of grafting vinyl monomers onto preirradiated
fluoropolymers. Irradiation of PVDF under vacuum and subsequent
exposure to atmospheric oxygen led to formation of surface oxides
and peroxides, which played a significant role during
peroxydisulfate initiated graft copolymerization of aniline.
FIG. 2 is a schematic diagram depicting surface modification of
PVDF polymer powder according to various embodiments. A chain
mechanism is involved due to formation of sulfate ion radicals
(SO.sub.4.sup.-.) which are ion chain carriers for the graft
copolymerization onto various polymer systems.
As shown in FIG. 3, sulfate ion radicals (SO.sub.4.sup.-.) are the
primary radicals generated from the ammonium persulfate (APS) by
the reduction of one electron. Simultaneously, APS generates
SO.sub.4.sup.-2 ions by the reduction of two electrons and acts as
an oxidant. They initiate the oxidative polymerization reaction of
aniline via a medium of cationic radicals and form PANI and PANI
radicals, as shown in FIG. 4. Lastly, PVDF macro radicals and PANI
cation radicals combine to form PVDF-g-PANI graft copolymer, as
shown in FIG. 5.
FIG. 6 shows the photographic images of 20-30 .mu.m of pristine and
polyaniline grafted (PVDF-g-PANI) films. It is clearly visible that
both the films are flexible with ease of handling for different
applications.
Example 8: Optimization of Graft Copolymerization Synthesis
Conditions
To optimize the conditions for grafting of PANI onto the
preirradiated PVDF, ammonium persulfate hydrochloric acid aniline
monomer reaction temperature and reaction time were varied keeping
total volume of the reaction mixture fixed at 50 ml. It was
observed that (NH.sub.4).sub.2S.sub.2O.sub.8/hydrochloric acid
system may be efficiently used in the graft copolymerization of
polyaniline on to PVDF, where maximum 11.35% grafting was achieved.
Optimum conditions for maximum percentage of grafting (11.35%) used
were: HCl concentration=0.9 mol/L; monomer concentration=0.1 mol/L;
temperature=55.degree. C.; time=5 hrs; initiator concentration=0.08
mol/L (see Table 1).
TABLE-US-00001 TABLE 1 Optimization of various reaction parameters
for maximum percentage graft copolymerization of aniline onto PVDF
polymer powder HCl Initiator concentration Time Monomer
concentration S/No. mol/L Temperature (.degree. C.) (Hrs) mol/L
.times. 10.sup.-2 mol/L % Grafting 1. 0.3 40 4 0.05 0.04 1.4 2. 0.6
40 4 0.05 0.04 2.3 3. 0.9 40 4 0.05 0.04 3.6 4. 1.2 40 4 0.05 0.04
3.3 5. 0.9 35 4 0.05 0.04 3.5 6. 0.9 45 4 0.05 0.04 4.4 7. 0.9 55 4
0.05 0.04 5.2 8. 0.9 65 4 0.05 0.04 4.9 9. 0.9 55 3 0.05 0.04 4.2
10. 0.9 55 4 0.05 0.04 5.5 11. 0.9 55 5 0.05 0.04 6.4 12. 0.9 55 6
0.05 0.04 6.4 13. 0.9 55 5 0.075 0.04 7.1 14. 0.9 55 5 0.1 0.04 7.7
15. 0.9 55 5 0.125 0.04 7.6 16. 0.9 55 5 0.1 0.02 4.5 17. 0.9 55 5
0.1 0.04 7.4 18. 0.9 55 5 0.1 0.08 11.35 19. 0.9 55 5 0.1 0.12
10.85
Example 9: Characterization of PVDF-g-PANI
The pristine PVDF and PVDF-g-PANI samples were characterized by
Fourier Transform Infrared Spectroscopy (FTIR), Thermogravimetric
Analysis (TGA) and Differential Scanning calorimetry (DSC)
analysis. The results clearly demonstrate that polyaniline was
successfully grafted into PVDF polymer.
Example 9.1: FTIR Spectral Study
The FTIR spectra of pristine PVDF, PANI homo polymer and
PVDF-g-PANI are shown in FIG. 7(a). In the spectrum of pristine
PVDF polymer, the characteristic peaks in the base polymer were
those near to v.sub.max 3000 cm.sup.-1 representing C--H stretching
vibration. The characteristic absorption bands at v.sub.max 1185
cm.sup.-1 and v.sub.max 1403 cm.sup.-1 were due to the absorption
peaks of CF.sub.2 stretching and CH.sub.2 stretching modes
respectively. The asymmetric and symmetric stretching vibrations of
the CH.sub.2 group in the pristine PVDF are located, respectively,
at v.sub.max 3026 cm.sup.-1 and v.sub.max 2973 cm.sup.-1. The FTIR
spectrum of the PVDF grafted PANI exhibited strong absorption
throughout the range 4000-500 cm.sup.-1. The FTIR spectrum shows
peaks at v.sub.max 1580 and v.sub.max 1492 cm.sup.-1 corresponding
to an aromatic benzenoid and quinoid ring. The peaks observed at
v.sub.max 1279 cm.sup.-1 indicate the C--N stretching vibration of
aromatic secondary amine. The peak corresponding to the
N.dbd.O.dbd.N bending vibration of PANI shifted to lower wave
number (v.sub.max 1072 cm.sup.-1 from v.sub.max 1108 cm.sup.-1).
The shift can be attributed to the hydrogen bonding between PVDF
and imine group of the grafted PANI chains. The peak at v.sub.max
833 cm.sup.-1 may be attributed due to the aromatic C--H bending
vibration band due to 1,4-disubstituted benzene ring. Other peaks
observed in the spectrum corresponded to the vibration spectrum of
PVDF backbone polymer.
For comparison, the FTIR spectrum of the PANI homo polymer was also
taken. The peaks at v.sub.max 1483 and v.sub.max 1567 cm.sup.-1
indicate the presence of ring stretch of benzenoid and quinoid
form. The presence of dopants, Cl.sup.- peak is evident from the
peak at 1307 cm.sup.-1. The peak appearing at 1252 cm.sup.-1 is due
to the C--H stretch of aromatic secondary amine. The peak at 815 cm
cm.sup.-1 is due to para linked benzene ring. The peaks at 887 and
772 cm.sup.-1 are due to the aromatic out of plane C--H bending
stretch. These assignments are in close agreement with the earlier
reports of FTIR studies of PANI. The FTIR study of the grafted
polymer hence adds evidence for the successful chemical grafting of
PANI onto PVDF.
Example 9.2: Thermal Analysis
The thermal stability of PVDF-g-PANI was determined by
thermogravimetric analysis (TGA) taking pristine PVDF as reference.
The TGA curves of PVDF-g PANI, pristine PVDF and polyaniline
homopolymer are shown in FIG. 7(b).
TGA study indicated that the pristine PVDF was stable up to about
400.degree. C. and the decomposition of the pristine PVDF onset at
437.degree. C. and suffered a weight loss of less than 5% at about
455.degree. C. This behaviour may be attributed to the degradation
of the pristine PVDF matrix. PVDF showed maximum weight loss
(around 70%) in the temperature range 450-600.degree. C. after
which it decreased slowly. Thermal study of PVDF-g-PANI showed that
the copolymer decomposition onset at 91.degree. C. with 62% weight
loss up to 600.degree. C.
The initial mass loss (5%) between 50 and 450.degree. C. in
PVDF-g-PANI was due to the loss of water and solvent in the polymer
chains. In the TGA profile, the weight loss at the higher
temperature indicated a structural decomposition of the polymer. A
weight loss of about 63-66% was observed in the temperature range
600-900.degree. C. for the grafted copolymer. In case of
homopolymer PANI, three stages of weight loss were observed. The
first weight loss around 14% was observed up to 200.degree. C.
which may be attributed to the loss of bound and adhered water; a
second weight loss of 30% followed up to 450.degree. C. The third
weight loss started at 467.degree. C. and about 47% weight was lost
up to 600.degree. C., after this point, it degraded slowly. A total
loss of 64% was observed up to 900.degree. C. These results
indicated that polyaniline was successfully grafted onto PVDF and
exhibited intermediate weight loss behaviour in comparison to that
of pristine PVDF and PANI.
Example 9.3: Differential Scanning Calorimetry
The DSC curves of the pristine PVDF powder, PVDF-g-PANI and PANI
are shown in FIG. 7(c). The melting point (T.sub.m) of pristine
PVDF was found to be 157.34.degree. C. with the melting enthalpy
(.DELTA. H.sub.m) 39.91 J/g. For PVDF-g-PANI the melting point
(T.sub.m) was found to be 155.80.degree. C. with the melting
enthalpy (.DELTA. H.sub.m) 33.23 J/g. For polyaniline homopolymer
the melting point (T.sub.m) and the melting enthalpy (.DELTA.
H.sub.m) were found to be 131.35.degree. C. and 262.6 J/g
respectively.
The percentage crystallinity (X.sub.C) of the pristine PVDF and the
PVDF-g-PANI were calculated from the heat of fusion and were found
to be 38.11 and 34.79 respectively. The graft copolymerization of
PVDF with polyaniline resulted in some obliteration in the
structural symmetry of PVDF ensuing in the lowering of percentage
crystallinity. It was postulated that the introduction of
polyaniline graft-chains diluted the crystalline zone of the PVDF
matrix and this phenomenon was probably associated with the
decreasing proportion of the PVDF crystalline phase in the grafted
polymer, possibly indicating the interception of polyaniline grafts
into the crystalizable part of the PVDF melt.
Example 10: Electrical and Dielectric Properties Measurements
Example 10.1: Electrical Properties Measurement
The electrical conductivity of synthesized graft copolymer films
(material with optimum percentage of grafting) was measured with
reference to pristine PVDF. FIG. 8 shows the frequency dependencies
of conductivity of the PVDF-g-PANI copolymer film. The conductance
of the pristine PVDF samples increased with frequency which was
characteristic of non-conductive materials. The conductivity of
PVDF-g-PANI film was almost frequency independent at low
frequencies (<1 KHz). This frequency independence was
characteristic of conductive materials, and was believed to be a
result of the grafting of polyaniline. This higher conductivity was
likely due to the incorporation of PANI grafts which increased the
formation of elongated and connected PANI domains within the
grafted network.
Example 10.2: Dielectric Properties Measurement
The dielectric behavior of pristine PVDF films and PVDF-g-PANI
copolymer films was studied at different temperatures and the
results were analyzed in terms of different parameters such as
dielectric constant (.di-elect cons.), dielectric loss tangent and
breakdown strength.
Both dielectric constant and loss depend upon two factors: (1)
dipole density, which is characterized by the total number of
dipoles per unit volume, and (2) the ability of the dipoles to
follow the reversal of polarity with applied electrical field,
i.e., the mobility of the dipole, which in turn depends upon the
mobility of the charges on the polymer chains to which the dipoles
are attached.
Like most polymers, PVDF is not electrically conductive. On the
other hand, PANI represents a class of quasi-one-dimensional
disordered conductors consisting of bundles of well coupled chains.
This is opposed to conventional quasi one-dimensional conductors
which have isolated chains. Within the bundles of well coupled
chains, the electron is three dimensionally delocalized. The
dielectric constants of PVDF homopolymer and PVDF-g-PANI copolymer
were determined by measuring the capacitance (C). The dielectric
constant was evaluated from the capacitance measurement using
Equation (1) .di-elect cons.=Cd/.di-elect cons..sub.0A Equation
(1)
where C is the capacitance with an isotropic material filling the
space, .di-elect cons..sub.0 is the permittivity of free space
which is equal to 8.85.times.10.sup.-12 f/m, A is the cross
sectional area of the sample, and d is its thickness.
FIG. 9(a) shows the dielectric constant of pristine PVDF and
PVDF-g-PANI as a function of frequency at room temperature. For
pristine PVDF, the dielectric constant decreased with increasing
frequency. This may be attributed to the tendency of dipoles in the
PVDF molecules to orient themselves in the direction of the applied
field in the low frequency range. However, at higher frequencies,
the dipoles are not able to respond to the rapidly changing
direction of the applied field which causes the value of E to
decrease.
On the contrary, the .di-elect cons. values of PVDF-g-PANI
copolymer (optimized grafting 11.35%) reduce rapidly over the whole
range of frequency (up to 10.sup.6 Hz) as compared to pristine
PVDF. Most noteworthy, as evident from FIG. 9(a), the dielectric
constant of the grafted polymer is significantly larger in
comparison with the pristine polymer. At 1 kHz, the dielectric
constant of PVDF-g-PANI film reaches about 1235, representing more
than 100 times increment as compared to pristine PVDF film. The
dielectric constant of the PVDF-g-PANI film is mainly enhanced by
the aniline monomer. The graft copolymerization synthesis reaction
results in the incorporation of the conductive conjugated aniline
polymer. This incorporation conjures the formation of
nanocapacitors within the PVDF-g-PANI film. These nanocapacitors,
which are impregnated throughout the PVDF-g-PANI film, may be
understood as regions of conductive aniline which surround regions
of insulating PVDF, thereby forming capacitor structures which
contribute to the overall capacitance of the film.
Dielectric constant has been found to increase with increase in
grafting (FIG. 9(b)) and this behaviour can be attributed to
incorporation of more conducting polyaniline into the pristine
PVDF. With the incorporation of more polyaniline as a result of
increasing percentage grafting, more microcapacitors to
nanocapacitors are formed contributing to the enhanced dielectric
constant. It has been observed that the dielectric constant
increases with percentage grafting up to the highest grafting
percentage of 11.35%. It is interesting to note that even at 11.35%
polyaniline grafting, there was no sharp increase in the dielectric
constant or dielectric loss tangent, indicating the percolation
threshold was still not achieved. In addition, the conductivity
measured (in FIG. 8) was relatively low at about 2.times.10.sup.-5
S/m. These results confirm that the polyaniline critical
concentration had not been reached, eluding the formation of
percolative paths, preventing a leaky conductive film.
The frequency dependences of the loss tangent for the pristine PVDF
and PVDF-g-PANI film samples are presented in FIG. 10(a). Albeit a
higher loss tangent for the grafted film in comparison to the
pristine PVDF, the PVDF-g-PANI loss tangent was comparable to PVDF
PANI nanohybrid films. It has been observed that the dielectric
loss for the grafted film was almost independent of frequency. The
dielectric loss was higher than the pristine PVDF polymers and
resulted from the conductivity caused by the network of long PANI
chains as well as some possible impurities left in the sample. The
loss tangent for PVDF-g-PANI film was also found to increase with
percentage of grafting (FIG. 10(b)). This is a result of increase
in conductivity caused by the connection of long conductive chains
of PANI.
Other factors such as greater fraction of the amorphous component
as well as less molecular orientation; disruption of packing of
rigid chain polymers; higher free volume as a result of
incorporation of bulky polyaniline chains with increase in grafting
percentage also contributed to the high dielectric loss.
FIGS. 11(a) and (b) shows the temperature dependence of dielectric
permittivity and dielectric loss for PVDF and PVDF-g-PANI copolymer
films. A broad peak may be readily seen at about 70.degree. C. for
the grafted film with optimized grafting, most probably due to the
coupled chains of polyaniline. At high temperatures (>70.degree.
C.), the motion of PVDF-g-PANI molecule chains were thermally
activated, thereby disrupting the conducting polyaniline network,
and hence, the nanocapacitors structure which led to a reduced
dielectric constant. For the dielectric loss, it generally
increased with elevated temperature as expected. Such a high
dielectric loss as compared to pristine PVDF was primarily caused
by the conductive polyaniline films. In fact, for both the pristine
PVDF and PVDF-g-PANI films, the dielectric loss was also governed
by the heterogeneity and motion of molecular chains. Since the
motion of molecule chains was temperature-activated, it manifested
itself by an increase in dielectric loss with temperature.
FIG. 12 shows the schematic of the Multi Layer Polymer (MLP)
capacitor. The 1 and 3 layer MLP showed a proportional increase in
the capacitance with the increase in area compared to the standard
1 mm diameter electrode samples. Note that the dielectric results
reported so far are of the standard 1 mm diameter electrode
samples. For the 1 layer MLP, the capacitance was found to be 5991
pF, which was approximately 16 times the capacitance of the 1 mm
electrode device (375 pF). This is close to the theoretical value
of 13 times (Total area of 1 layer MLP (10 mm.sup.2) Area of 1 mm
electrode device (0.79 mm.sup.2). Similarly, for the 3 layer MLP,
the capacitance was found to be 8.7 nF, which was approximately 23
times the capacitance of the 1 mm electrode device which was close
to the theoretical value of 25 times (Total area of 3 layer MLP (20
mm.sup.2) Area of 1 mm electrode device (0.79 mm.sup.2). The result
is shown in FIG. 13.
Example 10.3: Calculation of Maximum Energy Density
To have a maximum energy density, the grafting should be optimized
in a film in such a way that the combination of the effective
permittivity and the breakdown strength gives the maximum value of
energy density. The energy density of the grafted film was measured
by the standard polarization method. The PVDF-g-PANI films showed
promising energy density of 3.7 J cm.sup.3 at 1000 V. Therefore,
based on high dielectric permittivity and breakdown strength, the
PVDF-g-PANI hybrid films with optimum grafting (11.35%) afforded
large energy density. This value may be much higher depending on
the applied electric field across the capacitors. The values of the
energy density were greater than the current state of the art
BaTiO.sub.3 high-energy-density capacitors (3.5-4 J/cm.sup.3) at
the same applied voltage.
Example 10.4: Charge and Discharge Time for Grafted Polymer
FIG. 14 shows the discharge time against the various percentage of
grafting polyaniline onto the PVDF measured at 2.7 K.OMEGA.. The
discharge time increased slightly with increasing percentage of
grafting polyaniline onto the PVDF. Compared to the pure PVDF and
grafted PVDF films, the discharge time was in the range of 12-17
.mu.s, close to the pristine PVDF (9.4 .mu.s).
In brief, the graft copolymerization of polyaniline onto the
pristine PVDF had an imperative effect on the dielectric properties
of the PVDF-g-PANI films. At optimum grafting (11.35%), there was a
significant increase in the effective dielectric permittivity.
The trends for the permittivity and tangent loss over the optimum
grafting studied were reasonably well described by accounting for
nanocapacitors formation and enhanced conductivity due to more
formation of PANI elongated and connected domains within the
grafted network as a result of more incorporation of PANI grafts
under grafting conditions indicating that the grafting of PANI was
responsible for the enhanced dielectric properties in the PVDF-g
PANI films.
Grafting of polyaniline onto ferroelectric PVDF polymer films by
free radical induced graft copolymerization has been demonstrated.
Methods demonstrated herein are applicable for many polymer/monomer
combinations and unlike chemically initiated grafting; there was no
contamination form initiators.
The PVDF and PVDF-g-PANI copolymer films have been prepared by tape
casting method. The electrical and dielectric properties of the
grafted films have been investigated. The dielectric constant
obtained in the PVDF-g-PANI copolymer was much higher than that in
other polymer system reported so far. The dielectric constant of
1235 (at 10.sup.3 Hz) was obtained in the copolymer films with
11.35% grafting with dielectric loss of 0.12. The results of
electrical properties are shown in FIGS. 9 and 10.
It may be concluded that the high dielectric constant followed by
reduced loss tangent field of graft copolymer observed in the
copolymer films is partially caused by the nanocapacitors formation
and good compatibility between the two polymer systems. The
particular interest in grafted films lies due to high dielectric
constant and low loss, which makes these films well suitable for
possible application in high energy density capacitors. The success
of such efforts, as well as development of approaches to mitigate
dielectric breakdown in grafted polymer materials, could pave the
way to wide ranging applications that take advantage of the energy
storage potential and processability of conducting polymer grafted
ferroelectric polymer systems.
In addition, the multilayer polymer capacitor has been demonstrated
by PVDF-graft-polyaniline. The multilayer capacitor fabricated from
PVDF-g-PANI also shows the promising properties. It is believed
that more studies will be accomplished and remarkable performance
improvements will be achieved with the development of decent
syntheses and proper device fabrications.
* * * * *